Use another instance as the source of an instance (copy)

[stgraber]

This is somewhat related to #42 in that there are other ways to create instances.

A common one I find myself needing is the ability to create an instance from another one.

This would likely look something like:

resource "incus_instance" "this" {
    remote = "my-cluster"
    project = "my-project"
    target = "some-server"

    name = "foo"
    type = "container"

    source_instance {
        project = "source-project"
        name = "bar"
        snapshot = "snap0"
    }
}